Downmix

Downmix is the process of converting an audio signal with more channels into a lower-channel configuration, most commonly from multichannel formats such as 5.1 or 7.1 surround to stereo or mono. It is used to ensure compatibility with playback systems that do not support the original channel count and to provide a usable listening experience across a range of devices and formats.

The technique relies on a downmix matrix, a set of linear coefficients that determine how much of each input channel contributes to each output channel. In a typical 5.1 to stereo downmix, front left and front right channels are combined with inputs from the center and surround channels, while the low-frequency effects channel is often handled separately or blended at a modest level to avoid overpowering the main channels. The goal is to preserve spatial cues and overall balance while preventing clipping and excessive phase interactions that can degrade localization.

There are variations for different targets, such as stereo and mono downmixes, and some systems apply perceptual adjustments to retain dialog intelligibility or ambient cues. Downmixing can be performed during production and mastering, or in real time by playback devices and streaming engines to ensure broad compatibility.

Quality considerations include managing phase relationships, preserving sense of space, and avoiding artifacts in loudness or tonal balance. While no single standard governs all downmix practices, well-designed matrices aim to deliver coherent and natural-sounding results across diverse listening environments.